https://github.com/go-playground/validator/blob/v9/_examples/dive/main.go

validator.go

package main

import (
	"fmt"

	"gopkg.in/go-playground/validator.v9"
)

// Test ...
type Test struct {
	Array []string          `validate:"required,gt=0,dive,required"`
	Map   map[string]string `validate:"required,gt=0,dive,keys,keymax,endkeys,required,max=1000"`
}

// use a single instance of Validate, it caches struct info
var validate *validator.Validate

func main() {

	validate = validator.New()

	// registering alias so we can see the differences between
	// map key, value validation errors
	validate.RegisterAlias("keymax", "max=10")

	var test Test
	// test 1
	val(test)
	
	// test 2: nested struct
	test.Array = []string{"123451234512"}
	// vs: test.Array = []string{""}
	
	// validation 'dive' into k, v of the map
	test.Map = map[string]string{"12": "123451234"}  //key met max=10 requirement, change to exceeed this length for testing
	// vs: test.Map = map[string]string{"123451234512": "123451234"}
	val(test)
}

func val(test Test) {
	fmt.Println("testing")
	err := validate.Struct(test)
	fmt.Println(err)
}